Facebook users were more likely to eat unhealthy snacks after spending time on the social networking website, particularly if they had been in contact with close friends over the internet.

Interacting with friends online could raise people's confidence, which in turn reduces their powers of self control and makes them more likely to snack on unhealthy food immediately afterwards, researchers suggested.

The study illustrates how significant an effect social networking sites can have on society and people's behaviour, the researchers from Columbia and Pittsburgh Universities said.

Writing in the Journal of Consumer Research, they explained: "Using online social networks can have a positive effect on self-esteem and well-being. However, these increased feelings of self-worth can have a detrimental effect on behaviour.

"Because consumers care about the image they present to close friends, social network use enhances self-esteem in users who are focused on close friends while browsing their social network. This momentary increase in self-esteem leads them to display less self-control."

The scientists used five experiments on the behaviour of Facebook users to determine what effect their online activity was having on their offline behaviour, for example what they ate and how much money they spent.

The five studies suggested a link between use of social networks such as Facebook and poor self-control over one's health, mental persistence and spending.

People who spent most of their time online communicating with close friends had higher levels of self-esteem and were more likely to eat unhealthy snacks after logging off, for example.

The same was not true of people who primarily focused on "weak ties" they were less familiar with, the researchers reported.

Those who spent more time talking to close friends online also had on average a higher body mass index – a ratio of height to weight – and more credit card debt, results from the study showed.

The researchers said their results were "concerning" because of the increasing amount of time people spend on social networks, and the ease of access to sites like Facebook through gadgets like smartphones.

"Given that self-control is important for maintaining social order and personal well-being, this subtle effect could have widespread impact," they said.

"This is particularly true for adolescents and young adults who are the heaviest users of social networks and have grown up using social networks as a normal part of their daily lives."